There is provided a process for producing a styrenic compound by reacting an aromatic compound with an aldehyde over an intermediate pore size molecular sieve catalyst, such as a ZSM-5 catalyst.
There are several established ways to produce styrenes. Styrene is obtained by the dehydrogenation of ethylbenzene; p-methylstyrene can similarly be obtained from p-methylethylbenzene (p-ethyltoluene). This method, however, is not suitable when larger or reactive substituents are present. For example, n-propyl styrene can not be prepared by dehydrogenation of n-propyl ethylbenzene. Other means have therefore been sought for the synthesis of alkyl-substituted styrenes.
One common method used in these cases consists in reacting the aromatic compound with acetaldehyde at low temperature to form a 1,1-diaryl ethane which is subsequently cracked to form equimolar amounts of the styrene and the aromatic. For example, R. A. Innes and M. L. Ocelli, AIChE 1984 Summer Natl. meeting, prepr. #42d:27p, describe the reaction of toluene with acetaldehyde at 5.degree.-10.degree. C. with 98% sulfuric acid as catalyst to yield 1,1-ditolylethane; the latter is cracked at 525.degree. C. with an offretite type zeolite catalyst to yield equimolar amounts of p-methylstyrene and toluene. Similarly, EP 0168803, assigned to Nippon Petrochemical, discloses the reaction of acetaldehyde with isobutylbenzene with sulfuric acid at -20.degree. to 25.degree. C. to give 1,1-bis (p-isobutylphenyl) ethane, which is cracked with protonic acids at 200.degree.-650.degree. C. in the liquid phase (e.g., H.sub.3 PO.sub.4, H.sub.2 SO.sub.4) or at 300.degree.-650.degree. C. in the gas phase over clays, silica-alumina or zeolites to produce the desired p-isobutylstyrene together with isobutylbenzene that must be recovered and recycled.